Browsing by Author "Meng, HF"

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Bipolaron-induced third-order optical nonlinearity in conjugated conducting polymers
(1996-12-15)
Lai, CM
;
Meng, HF
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
A mechanism, due to the presence of bipolarons, for large resonant chi((3)) in doped cis-polyacetylene and other conjugated conducting polymers is presented. Under optical pumping, the electronic occupations of the two intragap energy levels of the bipolarons will vary, resulting in a shift in their energies, which in turn causes a dramatic change in the absorption spectrum. Even for moderate doping concentrations of bipolarons, chi((3)) is found to be as high as 10(-8) esu.
Bipolaron-induced third-order optical nonlinearity in conjugated conducting polymers
(1996-12-15)
Lai, CM
;
Meng, HF
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
A mechanism, due to the presence of bipolarons, for large resonant chi((3)) in doped cis-polyacetylene and other conjugated conducting polymers is presented. Under optical pumping, the electronic occupations of the two intragap energy levels of the bipolarons will vary, resulting in a shift in their energies, which in turn causes a dramatic change in the absorption spectrum. Even for moderate doping concentrations of bipolarons, chi((3)) is found to be as high as 10(-8) esu.
Bond length order parameter of conjugated polymers
(1999-11-01)
Lai, CM
;
Meng, HF
;
物理研究所
;
Institute of Physics
It is usually assumed that the ground state lattice configuration of polyacetylene is a dimerized chain. We found that the energy can be further reduced by an overall chain length contraction, which leads to a new bond length order parameter, in addition to the well-known bond alternation order parameter. For quasi-particles like solitons and polarons, these two order parameters are coupled with the electron wave functions and vary in space. We generalize the Takayama-Liu-Maki equations in the continuum limit to incorporate this new order parameter. Significant modifications are found for the polaron solution. Polyacene, a ladder polymer, is also studied with the inclusion of the bond length order parameter. The ground state is predicted to be a non-alternating structure, with zero energy gap and unusually high electric conductivity. (C) 1999 Elsevier Science B.V. All rights reserved.
Carrier generation mechanisms for photoconductivity in conjugated polymers
(2000-04-15)
Meng, HF
;
Hong, TM
;
物理研究所
;
Institute of Physics
We study theoretically the origin of photoconductivity in conjugated polymers. The charge carriers are released when excitons dissociate through the deep levels within the semiconductor bandgap caused by either oxidation (carbonyl group) or structure defects. Our results explain the recent observations of the temperature-independent photocurrent in the sweep-out regime and the transient photocurrent at high excitation. The tightly bound exciton versus semiconductor band controversy on the photocarrier generation process is clarified.
Color-tunable multilayer light-emitting diodes based on conjugated polymers
(2004-02-16)
Huang, CC
;
Meng, HF
;
Ho, GK
;
Chen, CH
;
Hsu, CS
Wide-range low-voltage continuous color tuning is achieved in multilayer light-emitting diodes based exclusively on the commonly used high-efficiency electroluminescent conjugated polymers. There are three layers for red, green, and blue emission, and one extra layer for electron blocking. The color of the emitted photon depends on the position of the electron-hole recombination. Due to the stronger field dependence of the electron mobility relative to the hole mobility, the recombination zone is pushed away from the cathode and concentrated in different emissive layers as the voltage increases. (C) 2004 American Institute of Physics.
Comment on "Triplet-to-singlet exciton formation in poly(p-phenylene-vinylene) light-emitting diodes" - Reply
(2004-02-06)
Lin, LC
;
Meng, HF
;
Shy, JT
;
Horng, SF
;
Yu, LS
Comment on "Triplet-to-singlet exciton formation in poly(p-phenylene-vinylene) light-emitting diodes" - Reply
(2003-11-21)
Lin, LC
;
Meng, HF
;
Shy, JT
;
Horng, SF
;
Yu, LS
Deep defect levels and exciton dissociation in conjugated polymers
(2001-09-01)
Meng, HF
;
Hong, TM
;
物理研究所
;
Institute of Physics
We study the localized deep electronic level within the bandgap of conjugated polymers in the presence of structure or oxygen (carbonyl group) defects. The structure defect is modeled as a chain twist, including the chain ends as a special case. Analytic expressions for both the energy and the wavefunction of the deep levels and the itinerant levels are obtained and supplemented by clear intuitive pictures. Carbonyl group is treated numerically within the tight-binding models. The rates of free carrier capture and exciton dissociation through the defect level via multi-phonon emission are calculated. We conclude that the defect dissociation dominates the intrinsic dissociation through thermal activation, and is the primary carrier generation mechanism in photoconductivity, Our results explain the photoconductivity enhancement due to oxidation, as well as the recent observation on the temperature-independent photocurrent in the sweep-out regime. (C) 2001 Elsevier Science B.V. All rights reserved.
Defect Auger exciton dissociation and impact ionization in conjugated polymers
(2001-09-15)
Chen, CH
;
Meng, HF
;
物理研究所
;
Institute of Physics
We study theoretically the exciton dissociation and carrier generation in conjugated polymers via defect Auger process, in which the electron (hole) of the exciton drops into the deep defect level while the energy is released to the hole (electron) through Coulomb scattering. Contrary to the usual Auger process among free carriers at high densities, defect Auger process for excitons takes place independent of the exciton density, and is identified as the dominant mechanism of photocarrier generation for excitation below the band gap. The dissociation probability for each passage through the defect is found to be close to one for excitons with thermal velocity, consistent with the picture that exciton decay in oxidized polymers is controlled by diffusion on a chain with quenching centers. We also study the reverse process, i.e., defect impact ionization, in which excitons or free electron hole pairs are created via the impact of hot holes (electrons) on electrons (holes) in the defect level. Excitons are found to be produced predominantly for driving electric field in a window around 10(5) V/cm along the chain. Light emission under unipolar carrier injection is predicted.
Efficient white light emission in conjugated polymer homojunctions
(2004-11-15)
Ho, GK
;
Meng, HF
;
Lin, SC
;
Horng, SF
;
Hsu, CS
We study polymer light-emitting diodes with a homojunction, i.e., junction between two layers with the same host material. One layer is poly (9,9-dioctylfluorene-2,7-diyl) (PFO) host blended with a small amount of poly (2-methoxy-5 (2'-ethyl-hexyloxy)-1,4-phenylene vinylene), another layer is either pure PFO or PFO host blended with green-emitting polyfluorene copolymers. Such homojunction devices are solution processed and show efficient white light emission. The peak luminance 3000 cd/m(2) is reached at 10 V with Internationale de L'Eclairage (CIE) coordinate (0.34, 0.34). (C) 2004 American Institute of Physics.
Electron traps in organic light-emitting diodes
(2005-06-01)
Tsai, MJ
;
Meng, HF
;
物理研究所
;
Institute of Physics
This work presents the effects of electron traps in organic light-emitting diodes using a model which includes charge injection, transport, and recombination. For electron-only devices, the electron current is reduced by the traps for several orders of magnitude at fixed voltage, and the traps strongly increase the transient time. For bipolar devices, due to negative trapped charges, traps enhance the hole current and the total current, opposite to the electron-only devices. The traps also make the recombination region close to the cathode. There is a voltage-dependent critical trap density beyond which the quantum efficiency decreases and transient time rises dramatically. The quantum efficiency is doubled if the hole traps are added to balance the electron and hole injections. Finally, the trap effect can be used in a bilayer light-emitting diode to make the emission color-tunable. (C) 2005 American Institute of Physics.
Electron-hole capture suppression and ultraviolet emission in conjugated polymers under high electric fields
(2002-06-15)
Chen, ES
;
Yeh, SH
;
Meng, HF
;
物理研究所
;
Institute of Physics
The probability for the Coulomb capture of an electron-hole pair to form an exciton in conjugated polymers is found to be significantly suppressed under high electric field, which is usually required for electroluminescence due to the low mobility. The Coulomb capture is a continuous descent of electronic energy through cascade emission of phonons, and the high field prevents this descent by heating up the carriers. On the other hand, ultraviolet emission through direct interband transition is found to be much less affected by the field, and becomes the dominant radiative decay channel for field beyond 10(8) V/m. Combined with impact ionization, our model provides a complete quantitative explanation of the peculiar high-field behaviors of the visible emission, ultraviolet emission, and current observed in conjugated polymer both before and after the avalanche breakdown.
Excimers in light-emitting conjugated polymers
(1998-08-15)
Meng, HF
;
物理研究所
;
Institute of Physics
The electronic structures of the low-lying excited states for two parallel chains of poly (p-phenylene-vinylene) with arbitrary relative positions are studied. The interchain interaction is treated microscopically. The energy and charge transfer fraction of the (light-emitting) lowest singlet state are found to depend sensitively on the horizontal shift of the two chains, which is in turn determined by the packing geometry. Our predictions of the excimer-forming geometry and the relative (dipole-forbidden) photoluminescence lifetimes are in excellent agreement with experiments.
Gain conditions for conducting polymer lasers
(1997-08-01)
Meng, HF
;
交大名義發表
;
物理研究所
;
National Chiao Tung University
;
Institute of Physics
The condition of lasing threshold for solid-state conducting polymers is studied using a model Hamiltonian, which includes quasi-particles corresponding to both Intrachain and interchain electronic excitations. We found that net gain can be achieved when the pump is so strong that intrachain and interchain species are mixed. A guide to overcome the difficulties coming from the photo-induced absorption due to the interchain excitations is proposed.
General method to solution-process multilayer polymer light-emitting diodes
(2006-04-17)
Tseng, SR
;
Lin, SC
;
Meng, HF
;
Liao, HH
;
Yeh, CH
An intermediate liquid buffer layer is introduced to overcome the dissolution problem of solution-processed multilayer conjugated polymer light-emitting diodes. This method can be applied to arbitrary combinations of polymers with no restriction on solvents. As an example, a hole-blocking layer is successfully spin coated on the common p-type emissive polymer layers. One green- and two blue-emitting polymers are chosen as the emissive layers. The electron-hole balance and efficiency are significantly improved by the addition of hole-blocking layer. The electroluminescence efficiency can be increased up to nine times, while the luminance up to seven times. In particular, 1.5 cd/A is obtained for deep blue emission from poly(9,9-dioctyl-fluorene) with 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene spin coated as the hole-blocking material. (c) 2006 American Institute of Physics.
Giant anisotropy of carrier mobility in electroluminescent conjugated polymer
(2006-01-09)
Meng, HF
;
Chen, BL
;
Tzeng, KC
;
Horng, SF
;
物理研究所
The horizontal and vertical hole mobility of amorphous spin-coated electroluminescent conjugated films are studied. It is found that the horizontal mobility is about four orders of magnitude larger than the vertical mobility. The horizontal mobility increases daily with the aging of the polymer solution from which the film is spun. Low temperature and photoluminescence study reveal that the giant mobility anisotropy is a result of the alignment of the aggregates, formed during aging, by the centrifugal force of the spin.
Intrachain carrier transport in conjugated polymer with structural and chemical defects
(2002-07-15)
Chen, YS
;
Meng, HF
;
物理研究所
;
Institute of Physics
By calculating the phase shifts of the wave functions for the extended scattering states within tight-binding model for a poly (p-phenylene vinylene) chain with one conjugation defect, we obtain the exact transmission probability through the defect as a function of the carrier incident energy for the entire eight pi bands. Cis-defect, sp(3) saturation, and oxidation are considered. The transmission increases rapidly from zero with the carrier kinetic energy, implying the conjugation breaks do not severely limit the intrachain charge transport under high electric field. Assuming an average conjugation length of 100 Angstrom separated by cis-defects, the drift velocity is predicted to be as high as 10(3) m/s for field at 10(7) V/m, and over 10(5) m/s at 10(8) V/m.
One-polymer active pixel
(2004-01-26)
Tzeng, KL
;
Meng, HF
;
Tzeng, MF
;
Chen, YS
;
Liu, CH
A metal-oxide field-effect transistor (MOSFET) based on an electroluminescent conjugated polymer is fabricated on a glass substrate. It is found that the mobility horizontal to the substrate is two to three orders of magnitude larger than the mobility vertical to the substrate. The high horizontal mobility is attributed to the in-plane chain alignment in amorphous spin-coated films. We demonstrate an active pixel in which the light-emitting diode and the driving MOSFET share the same active polymer. (C) 2004 American Institute of Physics.
Phonon sidebands and optical gain of light-emitting conjugated polymers
(1999-11-15)
Meng, HF
;
Chang, VCH
;
物理研究所
;
Institute of Physics
We study the interaction of excitons and optical phonons in light-emitting conjugated polymers with the variational approach. The optical transition matrix elements for the phonon sidebands in the absorption and emission spectra an calculated. The electron-phonon coupling constant is determined by fitting the theoretical oscillator strengths of the emission sidebands with the experimental photoluminescence spectra for the various poly(p-phenylene vinylene) derivatives. Using this coupling constant, we predict that the absorption one-phonon sideband is as important as the zero-phonon sideband for the ideal systems. Taking the experimental data of the peak absorption coefficient as input, we obtain the optical gain coefficient for a wide range of exciton densities. The gain is found to be determined by the exciton density no by a general expression 1.98 x 10(-16)n(o) cm(2), which is in good agreement with experiments. [S0163-1829(99)06243-8].
Polymer hot-carrier transistor
(2005-12-19)
Chao, YC
;
Yang, SL
;
Meng, HF
;
Horng, SF
;
物理研究所
Metal-base hot-carrier transistor with conjugated polymer emitter and collector is demonstrated. The device is fabricated by multiple spin coating with the metal base sandwiched between two polymers. A thin insulating layer of LiF is inserted between the emitter and base to enhance the hot carrier kinetic energy and reduce mutual dissolution. Using poly(9-vinylcarbazole) as the emitter, Al as the base, and poly(3-hexylthiophene) as the collector, common-emitter current gain of 25 is obtained with operation voltage as low as 5 V.